This invention is in the field of the synthesis of diacyl peroxide from acyl halide in liquid or supercritical carbon dioxide.
Diacyl peroxides are among the commonly used initiators in the commercial production of polyolefins, particularly fluoroolefins, such as tetrafluoroethylene. They may be represented as Rxe2x80x94(Cxe2x95x90O)xe2x80x94Oxe2x80x94Oxe2x80x94(Cxe2x95x90O)xe2x80x94R. The peroxide decomposes to give R., known as a free radical, which reacts with olefin monomer to begin the polymerization cycle. Taking tetrafluoroethylene as an example:
Rxe2x80x94(Cxe2x95x90O)xe2x80x94Oxe2x80x94Oxe2x80x94(Cxe2x95x90O)xe2x80x94Rxe2x86x922Rxe2x80x94(Cxe2x95x90O)xe2x80x94O.xe2x86x922R.+2CO2
R.+CF2xe2x95x90CF2xe2x86x92Rxe2x80x94CF2xe2x80x94CF2.
Rxe2x80x94CF2xe2x80x94CF2.+CF2xe2x95x90CF2xe2x86x92Rxe2x80x94CF2xe2x80x94CF2xe2x80x94CF2xe2x80x94CF2.
The R group arising from the initiator is called an xe2x80x9cendgroupxe2x80x9d of the polymer.
The classical synthesis of diacyl peroxides is an aqueous synthesis. An alkaline aqueous solution of hydrogen peroxide is contacted with a water-immiscible solution of acid halide. Examples are found in S. R. Sandler and W. Karo, (1974) Polymer Synthesis, Vol. 1, Academic Press, Inc., Orlando Florida, p. 451 and U.S. Pat. No. 5,021,516. This is a reaction of two liquid phases, an aqueous phase and a nonaqueous phase. Equation (1) shows the reaction:
2Rxe2x80x94(Cxe2x95x90O)X+H2O2+2NaOHxe2x86x92Rxe2x80x94(Cxe2x95x90O)xe2x80x94Oxe2x80x94Oxe2x80x94(Cxe2x95x90O)xe2x80x94R+2NaX+2H2Oxe2x80x83xe2x80x83(1)
From the stoichiometry of (1) it is clear that one mole of hydrogen peroxide reacts with two moles of acyl halide to yield one mole of diacyl peroxide. The acyl halide is added in a solvent that has low water solubility. The diacyl peroxide as it forms is taken up in the solvent. By this means, exposure of the acyl halide and the diacyl peroxide to the alkaline aqueous phase is minimized, which is desirable because water hydrolyzes both the organic acyl halide starting material and the diacyl peroxide product. Hydrolysis decreases yield and introduces byproducts such as acids and peracids, which are impurities. At the end of the reaction, the nonaqueous solvent with the diacyl peroxide dissolved in it is separated and dried, and purified as necessary.
Carbon dioxide (CO2) is among the most economical and environmentally benign nonaqueous solvents for polymerization. Polymerization in CO2 is simplified if initiator can be supplied in CO2. The use of diacyl peroxides in liquid or supercritical carbon dioxide is known (J. T. Kadla, et al., Polymer Preparation, vol. 39, no. 2, pp. 835-836, 1998). However, the peroxides were prepared using the aqueous alkaline peroxide method and were taken up in CF2Clxe2x80x94CFCl2 (CFCxe2x80x94113). Only then were they added to carbon dioxide.
A direct synthesis of diacyl peroxides in carbon dioxide is needed.
One form of this invention relates to a process for the synthesis of diacyl peroxide comprising contacting organic acyl halide with peroxide complex, in liquid or supercritical carbon dioxide.
A second form of this invention relates to a process for the continuous synthesis of diacyl peroxide comprised of continuously contacting a feed stream comprised of organic acyl halide in liquid or supercritical carbon dioxide with a bed comprised of peroxide complex, to form a product stream comprised of diacyl peroxide in liquid or supercritical carbon dioxide.
The present invention relates to the synthesis of diacyl peroxide in liquid or supercritical carbon dioxide by contacting organic acyl halide with peroxide complex in a medium of liquid or supercritical carbon dioxide. As stated above, the usual synthesis of diacyl peroxides is by reaction of aqueous alkaline peroxide with acyl halide. Surprisingly, it has been found that carbon dioxide, a Lewis acid, is an effective solvent for the production of diacyl peroxide by the reaction of acyl halide with peroxide complex. In addition, in a preferred form of the invention, liquid or supercritical carbon dioxide containing the resulting diacyl peroxide is collected as a product of the reaction. This mixture can be directly used in other processes, e.g., initiator supply for polymerization in carbon dioxide. This form of the invention provides a route to the direct synthesis in good yield of diacyl peroxides in carbon dioxide, minimizing the presence of water and eliminating any other organic solvent as would be inevitable in synthetic routes that would prepare the diacyl peroxide first in another solvent, and subsequently replacing that solvent, by whatever means, with carbon dioxide.
Organic acyl halides are compounds of the structure Rxe2x80x94(Cxe2x95x90O)X. X represents halogen: fluorine, chlorine, bromine, or iodine. The most readily available acyl halides are generally acyl chloride or acyl fluoride. R represents any organic group that is compatible with one or more of the peroxide complexes useful for carrying out this invention under the conditions of the synthesis. A compatible R group is one that does not contain atoms or groups of atoms that are susceptible to oxidation by or otherwise react with the other ingredients in the course of the reaction or in the reaction mixture to give undesirable products. R groups acceptable in the present invention include aliphatic and alicyclic groups, these same groups with ether functionality, aryl groups and substituted aryl groups in which the substituents are compatible with one or more of the peroxide complexes of this invention under the conditions of the synthesis. The R group may be partially or completely halogenated. If perhalogenated, the R group may have only one type of halogen, as with perfluorinated groups, or may have several types, as with, for example, chlorofluorinated groups.
The R group may also contain certain functional groups or atoms such as xe2x80x94COOCH3, xe2x80x94SO2F, xe2x80x94CN, I, Br, or H. The R group is incorporated in the polymer at the end of the polymer chain, that is, as an endgroup. It is sometimes useful to be able to further react the polymer through the endgroup with other molecules, for example, other monomers or polymer, or to introduce ionic functionality in the endgroup for interaction with polar surfaces such as metals, metal oxides, pigments, or with polar molecules, such as water or alcohols, to promote dispersion. Some of the functional groups above, for example xe2x80x94COOCH3 and xe2x80x94SO2F (the fluorosulfonyl group), are susceptible to hydrolysis, especially base-catalyzed hydrolysis, and reaction with nucleophiles. However, because of the absence of an aqueous phase in a preferred form of this invention and of the specificity of the peroxide complexes useful in carrying out this invention, these functional groups are not affected and the diacyl peroxides corresponding to these acyl halides can be made. For example, from FSO2CF2(Cxe2x95x90O)F, FSO2CF2(Cxe2x95x90O)xe2x80x94Oxe2x80x94Oxe2x80x94(Cxe2x95x90O)CF2SO2F can be made without hydrolysis of the sulfonyl fluoride functionality to sulfonic acid. It is a further advantage of the process according to this invention that such hydrolysis-sensitive groups can be incorporated in diacyl peroxides and thereby introduced as endgroups in polymers.
In the synthesis of diacyl peroxide in accordance with this invention, no more than one organic acyl halide will normally be used. Although with more than one organic acyl halide the reaction would proceed satisfactorily, more than one diacyl peroxide would be made. For example, if two organic acyl halides are used, Axe2x80x94(Cxe2x95x90O)X and Bxe2x80x94(Cxe2x95x90O)X, three diacyl peroxides would be expected: Axe2x80x94(Cxe2x95x90O)xe2x80x94Oxe2x80x94Oxe2x80x94(Cxe2x95x90O)xe2x80x94A, Bxe2x80x94(Cxe2x95x90O)xe2x80x94Oxe2x80x94Oxe2x80x94(Cxe2x95x90O)xe2x80x94B, and Axe2x80x94(Cxe2x95x90O)xe2x80x94Oxe2x80x94Oxe2x80x94(Cxe2x95x90O)xe2x80x94B, a mixed diacyl peroxide. The ratio of the peroxides can be controlled to some extent by the relative concentrations and order of addition of the organic diacyl halides. Such a mixture of peroxides is usually undesirable because the different peroxides will generally have different decomposition rates. However, if a mixed diacyl peroxide is wanted, the process according to this invention may be used, followed if necessary by separation or purification steps to reduce or remove the accompanying unwanted peroxides.
Diacyl peroxides in which the acyl group is a hydrocarbon group can be made according to this invention. These hydrocarbon diacyl peroxides are useful for initiation of olefin polymerization, including fluoroolefin polymerization when the presence of a hydrocarbon endgroup is acceptable or desirable. Isobutyryl peroxide is preferred when a low temperature hydrocarbon initiator is needed. It can be made from isobutyryl halide, preferably isobutyryl chloride.
Synthesis of diacyl peroxides according to this invention is particularly useful for making initiators for the polymerization of fluoroolefins such as tetrafluoroethylene, hexafluoropropylene, perfluoro(alkyl vinyl ethers), chlorotrifluoroethylene, vinylidene fluoride, and vinyl fluoride, either as homopolymers, or as copolymers with each other or with other olefins, such as ethylene and perfluoroalkylethylenes. Fluoroolefin polymerization is susceptible to chain transfer if compounds with labile carbon-hydrogen bonds are present, so it is desirable that initiators be free of such bonds. Furthermore, because of the high temperatures at which fluoropolymers are processed and used, the thermal and hydrolytic stability of the polymer endgroups is important. The R group of the initiator is one source of such endgroups. Therefore, except in cases where specific reactivity of polymer endgroups is wanted, in the interest of minimizing chain transfer activity of the initiator and of providing endgroups with thermal and hydrolytic stability comparable to that of the polymer chain, it is desirable that the R group be free of bonds that are capable of chain transfer or that are less thermally or hydrolytically stable than the polymer itself. In polymerizing fluoromonomers, perhalogenated R groups, and preferably perfluorinated R groups, meet this requirement. Because ether functionality in halogenated and fluorinated organic groups has good thermal and oxidative stability if the oxygen is between carbon atoms that are perhalogenated or perfluorinated, or between carbon atoms that are substituted with perhaloalkyl or perfluoroalkyl groups, such ether functionality is acceptable also.
It is a further advantage of diacyl peroxide synthesis in accordance with this invention that fluoroorganic acyl halides, that is, acyl halides in which the R group is at least partially fluorinated, and particularly perfluoroorganic acyl halides, are readily reacted to form the corresponding diacyl peroxides. An example of a perfluoroorganic acyl halide useful for this invention is perfluoro(2-methyl-3-oxa-hexanoyl fluoride), also known as hexafluoropropylene oxide (HFPO) dimer acid fluoride and as DAF. It has the formula:
CF3CF2CF2OCF(CF3)(Cxe2x95x90O)F
Other suitable perfluoroorganic acyl halides include CF3CF2CF2(Cxe2x95x90O)Cl (heptafluorobutyryl chloride) and CF3CF2(Cxe2x95x90O)F (pentafluoropropionyl fluoride).
The peroxide complexes useful for carrying out this invention include a) complexes of hydrogen peroxide with inorganic compounds, referred to here as inorganic complexes, and b) complexes of hydrogen peroxide with organic molecules, referred to here as organic peroxide complexes. These complexes include those substances in which hydrogen peroxide is combined with inorganic or organic compounds by bonds strong enough to permit isolation of the compounds, though the bonds may be weaker or of a different character than those between the constituents of hydrogen peroxide or of the compound with which it is complexed. By this criterion it can be seen that xe2x80x9csodium percarbonatexe2x80x9d, which is isolable and has the composition Na2CO3.1xc2xdH2O2, is a complex of hydrogen peroxide, while an aqueous solution of hydrogen peroxide, although it may have degrees of hydration that vary with concentration, is not. Complexes, as the term is used here, also include compounds such as sodium perborate, in which the elements of peroxide are reported to be an integral part of the molecule. The complexes according to this invention do not include persulfates or monopersulfates, such as potassium monopersulfate (KHSO5), which are found to be ineffective. It is believed that the stability oxygen-sulfur bond in the persulfate is so great that persulfates cannot provide the elements of hydrogen peroxide needed for this synthesis. Apart from these stipulations, nothing is implied as to the structure of the complexes. They may be combinations of hydrogen peroxide with the inorganic compound or organic molecule in which the peroxide is associated through weak or strong bonds. Alternatively, they may be reaction products of peroxide with the compound or molecule, in which elements of the peroxide are incorporated in the structure of the compound or molecule, but are available for reaction with acid halides. For some complexes, the structures may be unknown. It is preferable that the complexes be dry. It is more preferable that the complexes be anhydrous. The term xe2x80x9cdryxe2x80x9d means essentially free of water, though water of crystallization may be present. xe2x80x9cAnhydrousxe2x80x9d means essentially free of water including water of crystallization. A number of peroxide complexes and their syntheses are described in U.S. Pat. No. 5,820,841.
It is preferred for the peroxide complex to be substantially insoluble in liquid or supercritical carbon dioxide and to be present during the reaction as a solid phase. Such peroxide complexes are easily removed after reaction by filtration or used in the form of a bed through which the acyl halide in liquid or supercritical carbon dioxide is passed. Similarly, it is also preferred that the spent complex after reaction remain insoluble and in the solid phase.
Among the convenient inorganic peroxide complexes for the synthesis of diacyl peroxides according to this invention are percarbonate and perborate salts. These are most readily available as the sodium salts, which are used in the detergent industry. The other alkali metal salts of percarbonate or perborate, as for example, the potassium salts, may also be used in accordance with the processes of this invention. Those skilled in the art will recognize that the alkaline earth percarbonates and perborates, as for example, the calcium salts, though less desirable because less readily available, would be expected to be useful according to the processes of this invention. For the purposes of this invention, although both the alkali metal and alkaline earth percarbonates and perborates have utility in the synthesis of diacyl peroxides, the alkali metal salts are preferable, and the sodium salts are more preferable. For convenience, the percarbonate salts and perborate salts will be referred to herein simply as percarbonate and perborate.
Sodium percarbonate, Na2CO3.1xc2xdH2O2, is hydrolyzed by moisture, and for best results in the synthesis of diacyl peroxide according to this invention, the percarbonate should be kept dry. Sodium perborate, though represented as NaBO3.H2O and sometimes called sodium perborate monohydrate, is reported to be Na2(B2O8H4), and is therefore an anhydrous salt. Analogously, the so-called sodium perborate tetrahydrate is reported to be the trihydrate: Na2(B2O8H4).3H2O. The misnamed sodium perborate monohydrate is the preferred form to be used in the practice of this invention.
The organic peroxide complexes of this invention include those that may have some solubility in carbon dioxide, or at least be volatile enough to make separation from carbon dioxide difficult. The preferred organic complexes are those that are insoluble and whose residues are insoluble in carbon dioxide, and which are present during the synthesis as a solid phase. As such, they are easily separated from the diacyl peroxide solution. It is further desirable that the organic complexes be free of labile atoms or groups, or of bonds that can react with the reactants or products of the processes according to this invention, especially if such reactions degrade the organic molecule and such degradation products get into the reaction mixture.
Urea/hydrogen peroxide adduct (urea.H2O2) is a more preferred organic peroxide complex. It is commercially available (Aldrich Chemical Co. Milwaukee, Wis., USA). It is a solid and is essentially insoluble in the solvents designated herein and should small amounts be carried through filters or by other means into the diacyl peroxides solution, urea, not being active toward free-radical chain transfer, will have little or no effect on polymerization.
A significant advantage of organic peroxide complexes is that they introduce no metal ions into the reaction mixture and therefore give diacyl peroxide free of metal ions derived from the reactants. In polymerization, such diacyl peroxide made from organic peroxide complexes will introduce no metal ions into the polymer. Polymers, especially fluoropolymers, of low metal content, or free of metal ions, are needed for certain applications where high purity is required, such as the semiconductor industry.
An important characteristic of percarbonate, perborate, and urea/hydrogen peroxide adduct of this invention, and of the carbonate, borate, and urea remaining after the reaction, is their insolubility in carbon dioxide and because they are in the solid phase during the synthesis. Because they are solids, they can be easily separated from reaction mixtures by filtration. For the same reason, percarbonate, perborate, and urea/hydrogen peroxide adduct may be used in fixed beds for continuous synthesis of diacyl peroxides.
The temperature of the reaction is chosen to balance the interest in having a fast reaction with the need to prevent excessive loss of diacyl peroxide through thermal decomposition. Because diacyl peroxides vary in half-life (the time for one-half of the diacyl peroxide to be consumed; half-life is a function of temperature) reaction temperatures will vary, but useful temperatures are in the range of about xe2x88x9240xc2x0 C. to about 40xc2x0 C. For peroxides such as HFPO dimer peroxide, heptafluorobutyryl peroxide, isobutyryl peroxide, and bis[perfluoro(fluorosulfonyl)acetyl] peroxide, a temperature range of about xe2x88x9220xc2x0 C. to about 20xc2x0 C. is typical, about xe2x88x9210xc2x0 C. to about 10IC is preferred, and about xe2x88x925xc2x0 C. to about 5xc2x0 C. is more preferred when sodium percarbonate or sodium perborate is used. When urea/hydrogen peroxide adduct is used to make these diacyl peroxides, about xe2x88x920xc2x0 C. to about 10xc2x0 C. is the more preferred temperature. Diacyl peroxide loss to thermal decomposition is best minimized by keeping reaction time a fraction of the diacyl peroxide""s half-life at reaction temperature. A reaction time no greater than one-quarter of the diacyl peroxide half-life at the reaction temperature is preferred.
Because residual acyl halide is an impurity in the product diacyl peroxide, and is furthermore a source of acid that can cause corrosion, it is desirable to conduct the synthesis so as to yield as much of the diacyl peroxide as possible. Yield is preferably at least about 25%, more preferably at least about 50%, more preferably still at least about 70%, and most preferably at least about 90%.
The carbon dioxide used as solvent according to this invention will be in the liquid state at the preferred reaction temperatures for the synthesis of preferred diacyl peroxides. However, if it is desired to run the reaction at temperatures above the critical temperature of carbon dioxide, 31xc2x0 C., that can be done, in which case carbon dioxide in its supercritical state.
When diacyl peroxide is synthesized according to this invention in a batchwise manner, the reactant organic acyl halide is mixed with peroxide complex in a vessel containing a medium comprised of carbon dioxide. Surprisingly, it is found that the yield of diacyl peroxide increases as the mole ratio of peroxide in the peroxide complex to acyl chloride increases. It is preferable that the mole ratio be at least about one to one. It is more preferable that the mole ratio be at least about two to one. It is most preferable that the mole ratio be at least about four to one. Because the peroxide content of the peroxide complex depends upon the nature of the complex, the weight of complex that contains a mole of peroxide or its equivalent will depend upon the composition of the complex under consideration.
To prepare diacyl peroxide in a continuous reaction according to this invention, a feed stream comprised of organic acyl halide in liquid or supercritical carbon dioxide is continuously contacted with a bed comprised of peroxide complex to form a product stream comprising diacyl peroxide in liquid or supercritical carbon dioxide. The bed may be in the form of a column filled with peroxide complex and optionally an inert material. The purpose of the inert material would be to facilitate flow and temperature control. As stated above, the synthesis should be run so as to achieve high yield of the diacyl peroxide. The continuous method is preferred because it allows diacyl peroxide to be made as needed and consumed promptly. If desired, the diacyl peroxide in the liquid or supercritical carbon dioxide can be collected and advantageously used directly in that form. The continuous process ensures that fresh diacyl peroxide is always available and eliminates the need for diacyl peroxide storage, which generally requires low temperatures, and is therefore vulnerable to power outages and equipment failure. Furthermore, as with any oxidizing agent, it is sound practice to minimize the quantities of diacyl peroxide kept on hand. Both batch and continuous methods are demonstrated in the Examples.
Diacyl peroxide made according to this invention may be used in carbon dioxide to initiate polymerization. However, it is one of the advantages of making the initiator in carbon dioxide that the initiator may be conveniently transferred to another solvent by adding the initiator in carbon dioxide to said solvent and letting the carbon dioxide vaporize away. Any traces of carbon dioxide remaining can be removed if necessary by sparging, for example with nitrogen, or under reduced pressure. Using this xe2x80x9csolvent transfer methodxe2x80x9d, diacyl peroxide solutions of any desired concentration can be safely and easily made, even in solvents that would not be suitably used in the synthesis of the diacyl peroxide. Thus, the diacyl peroxide synthesis in carbon dioxide according to this invention can be the source of initiator solutions in a wide variety of solvents.